Flow properties of freeze-concentrated skim milk

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ELSEVIER

Flow Properties of Freeze-concentrated

Skim Milk

Yi-Huang Chang & Richard W. Hartel* Department

of Food Science, University of Wisconsin,

(Received 8 February

Madison. WI 53706. USA

1095: revised 9 August 1996; accepted 20 September

lY96)

ARSTRA CT The ,flow properties of freeze-concentrated skim milk, determined with a concentric cylinder viscometer; were well described by the simple power-law rheological model over the temperature range from -3.4 to -120°C. High-totalsolids freeze-concentrated skim milk (3.5-4070 total solids) exhibited non-Newtonian hehaviour; while lower concentrations gave Newtonian flow properties. The magnitude of the flow behaviour index was between 0.8 and 1.0. The Arrhenius model described well the effect of temperature on apparent viscosity in this temperature range. Statistical analysis showed that there was no difference in viscosity between condensed thermally evaporated skim milk and freeze-concentrated skim milk. However; the viscosities of reconstituted skim milks were different: the reconstituted product made from evaporated milk had a slightly higher viscosity than that of freeze-concentrated milk. 0 1997 Elsevier Science Limited. All rights reserved

NOMENCLATURE

a1 a2 C 5,

K n RZ

Constant in exponential model of the effect of concentration on apparent viscosity (dimensionless) Constant in power-law model of the effect of concentration on apparent viscosity (dimensionless) Concentration of total solids in milk solution (g/mL) Activation energy (kcal/mol) Consistency index (N s”/m’) Flow behaviour index Percentage of variability explained by the regression which is adjusted by taking into account numbers of degrees of freedom.

‘To whom correspondence

should be addressed 37s

(Tel: 608 263-1965).

Yi-Huang

376

ss T

TS

Chang, R. W Hartel

Sum of squares of the deviation between values Temperature (K) Total solids content in milk solution (wt%)

the data and the predicted

Greek symbols i

r?Zi VI0 VI v2

rlr VIZ (T 2

Shear rate (s-l) Apparent viscosity (mPa s) Constant in Eilers equation (mPa s) Constant in exponential model of the effect of concentration on apparent viscosity (mPa s) Constant in power-law model of the effect of concentration on apparent viscosity (mPa s) Reduced viscosity = ~JQ, Limiting viscosity in Arrhenius model (mPa s) Shear stress (dyn/cm2) Yield stress (dyn/cm2) Volume fraction of the disperse phase Maximum volume fraction in Eilers equation Limiting volume fraction in the equation of Krieger and Dougherty [eqn

WI INTRODUCTION In freeze concentration, water is separated from liquid food by crystallizing ice at low temperatures, followed by a separation step to remove ice from the concentrate. Due to the low temperatures of operation, no heat-induced changes occur, resulting in high-quality products for many liquid foods. Freeze concentration has been particularly successful in the concentration of citrus juices, but also finds applicability for coffee and tea extracts, beer and wine (Deshpande et al., 1984; Chowdhury, 1988). The application of freeze concentration to the dairy industry has been demonstrated in the past (Van Mil & Bouman, 1990); however, only limited commercial success has been obtained. It has been claimed that reconstituted skim milk previously concentrated by freeze concentration has a smoother and creamier product texture than the original skim milk (Anon., 1991). No scientific evidence for this claim has been published, although physical changes in protein structure may cause some organoleptic changes. This physical change can be evaluated objectively by comparing the viscosities of the natural skim milk and the reconstituted skim milk from freeze concentration processes. An important consideration in optimizing ice crystal growth in freeze concentration processes is the provision of rapid ice crystallization. Maintaining high rates of heat removal from the growth tanks results in rapid ice crystal growth (Shi et al., 1990). By varying the appropriate operating conditions, high rates of heat transfer can be maintained, resulting in these rapid rates of ice crystal growth (Hartel & Espinel, 1993). A better understanding of the flow properties of freeze-concentrated skim milk is fundamental to the control of heat and mass transfer rates between the ice and the liquid interface, which is necessary for design of operations related to the freeze concentration process, in sensory analysis and in quality control,

The flow properties of freeze-concentrated skim milk have never been reported, though the flow behaviour of skim milk, either natural or thermally evaporated, has been documented. The viscosity of skim milk as a function of the volume fraction is well described by the Eilers equation (Snoeren et al., 1982, for & between 0.5 and 0.75; Walstra & Jenness, 1984):

(1) or by the equation and 0.6):

of Krieger and Dougherty

(Griffin et al., 1989 for 4 between

0

rlr=(l-xC)P”

(2) with XC = (b/4’,, and 1) = 2.5d1,. As to processed skim milk, Langley & Temple (1985) presented a model for heated skim milk (TS between 1% and 10% heated to 100, 120 and 140°C) by expressing the relative viscosity in the form of the percentage of total solids: I\;, = JII exp(a, C) (3) The work of Hayashi & Kudo (1989) also showed that the viscosity of condensed skim milk (TS 44% to 50%) increased exponentially with increasing percentage of total solids and such condensed milk exhibited non-Newtonian behaviour. Recently, Reddy & Datta (1994) reported the effect of temperature (35 to 65°C) and concentration (TS 40% to 70%) on the consistency coefficient and flow behaviour index of reconstituted whole milk powder. The flow behaviour index of concentrated milk decreased with increasing concentration, but was not modelled as a function of temperature. The consistency coefficient decreased with increasing temperature according to the Arrhenius equation and decreased exponentially with concentration. Jeurnink & de Kruif (1993) suggested that the increase in viscosity of reconstituted heated skim milk was a result of an increase in size of casein micelles. In this study, an attempt was made to gain a broader understanding of the flow properties of freeze-concentrated skim milk by comparing the rheological properties of skim milk and freeze-concentrated and thermally evaporated skim milk.

MATERIALS

AND METHODS

Fresh skim milk was obtained from the University of Wisconsin dairy plant for preparation of freeze-concentrated skim milk solutions. The total solids content of the fresh skim milk was 9.56%. A 2 L stainless steel vessel with a cooling water jacket was used as a batch crystallizer to produce freeze-concentrated skim milk with different TS contents, using the technique described by Hartel & Espinel (1993). Refrigerated ethylene glycol solution from a constant temperature bath was circulated through the crystallizer jacket to maintain constant temperature conditions. A stirrer comprising three propeller blades (3.8 cm diameter) turning at 500 rev/min maintained agitation in the crystallizer. Liquid phase concentration was measured with a Bausch and Lomb Abbe 3-L refractometer (Rochester, NY). A small sample of ice-free‘ fluid was dropped on to the refractometer prism, and the refractive index was determined.

378

Yi-Huang Chang, R. W Hartel

Skim milk concentration was read from a calibration curve, generated by plotting the refractometer readings of different concentrated milk samples versus their true TS values determined by the standard oven drying method (AOAC, 1990). Ice crystal nuclei, used for the seeds in batch crystal growth studies, were formed in 10% lactose by spontaneous nucleation in a 600 mL jacketed vessel. The glycol refrigerant was cooled to -40°C and pumped through the nucleator jacket containing lactose solution. Once nuclei had formed, they were allowed to grow for 10 min under these conditions. Part of this slurry was filtered to separate ice crystal seeds from the lactose solution. These nuclei were added to the skim milk, held at the appropriate temperature in the growth crystallizer, to initiate the batch freeze concentration process. Crystallization was stopped when the liquid phase reached the desired TS, and liquid was separated from ice crystals by filtration. Viscosity data on freeze-concentrated skim milk were obtained with either glass capillary viscometers (Cannon-Fenske Routine Viscometer, No. 50 and No. 150) or a concentric-cylinder rotational viscometer (Brookfield LV series, LVTDV-II). Calculation of flow properties was based on the instruments and the particular instrumental constants. For the rotational viscometer, two cylindrical spindles (Brookfield LV series: No. 1 LV CYL, No. 2 LV CYL) were used together with the guard leg (diameter 81 mm). The calculation of shear rates followed the procedures provided, integrating the geometric factors mentioned above. The shear stresses were obtained by multiplying the readout percentage with the full-scale spring torque (673.7 dyn/cm2 for the LV system). To control temperature in the capillary viscometer, it was immersed in a 1.5 L glass vessel containing refrigerant solution. In the cylindrical viscometer, a 600 mL glass vessel with a cooling water jacket (i.d. 21.0 cm, o.d. 22.2 cm) was used to control the temperature of the milk contained within. A thermistor probe in the milk was used to monitor bulk temperature to within O.l”C. Viscosities of TS 9.56, 15, 20, 25, 30, 35 and 40% freeze-concentrated skim milk were measured at the freezing point, 0°C 5°C 10°C and 20°C. Freezing point was measured with an osmometer (Advanced Instruments Inc., Model 3W2) with an accuracy of f O.Ol”C. In addition, viscosity of skim milk produced by thermal evaporation in the UW Dairy Plant was measured at high concentration (35%). Freeze-concentrated skim milks (TS 15% and 35%) were reconstituted to 9.56% by addition of distilled water and compared with TS 9.56% fresh skim milk. The TS 35% thermally condensed skim milk was also reconstituted to TS 9.56% for comparison. The viscosity of each sample was measured on four different days and thus four different batches with five replicates for each batch were measured. One-way ANOVA was used to differentiate the effects of these processes (SAS Institute Inc., SASETAT, Cary, NC). Both simple power-law and Herschel-Bulkley models were used to describe the shear rate-shear stress data for the high-concentration skim milk: Power-law: Herschel - Bulkley:

(T= Kjf’ 0 = o,,+Kf”

(4) (5)

where G,, is the yield stress, K is the consistency index (N s”/m2) and n is the flow behaviour index. Linear and non-linear regressions were performed using the SAS package. The effect of temperature on viscosity was determined from the Arrhenius relation, which has been used extensively for quantifying the effect of temperature

.{I’)

Flow properties of concentrated skim milk

on flow properties model is given by:

of fluid foods (Holdsworth,

1971; Rao,

1977). The Arrhenius

rl,l = ‘I I exp(E,,IRT)

(6)

where Q, is the apparent viscosity (mPa s), q, is a parameter that is considered as the viscosity at infinite temperature (mPa s), EL, is activation energy (kcal/mol), R is the molar gas constant (kcal/mol K), and T is temperature (K). Apparent viscosity, )I;%,is defined as the ratio of shear stress, G, to shear rate, (j). The effect of concentration on viscosity was fitted by the exponential relation as in eqn (3) (Rao et al., 1984; Langley & Temple, 1985) and by the power-law relation (Rao et al., 1981): Power law:

?j;,= JT2.ci’J

(7)

where ye, is the apparent viscosity (mPa s) and C is the concentration of total solids in the milk solution (g/mL); ql, y2 (mPa s) and a,, a2 (dimensionless) are parameters determined by performing regression on data of viscosity versus concentration. RESULTS

AND DISCUSSION

The freezing points of the skim milk are listed in Table 1. Because of the limitations of the viscosity measurement systems, the actual operating temperatures for the viscometers were set slightly lower, as shown in Table 1. Viscosities for freeze-concentrated skim milk at the freezing point are compared in Tables 2 and 3 for low and high concentrations, respectively. The viscosity of 40% TABLE 1

Freezing

Point and Actual Measurement Temperature for Viscosities trated Skim Milk Samples

9.56

15 20 25 30 3.5 40

of the Freeze-concen-

Freezing point (“C)

Measurement temperature (“c‘)

- 0.54 + 0.01 -0.92&0.02 - 1.25rfI0.02 -1.73+0.06 -2.21 f0.04 ~ 2.78 f 0.06 -3.38+0.07

-0.6 - 1.0 - 1.3 - 1.8 -2.3 -2.8 -3.4

TABLE 2

Viscosity of Low-concentration

9.56 15 20

Freeze-concentrated

Skim Milk (Capillary

Viscometer)

Freezing Point (“C)

Viscosity (mPa s)

-0.6

3.70 * 0.01 6.59 f 0.08 11.9_+0.1

-1.0 -1.3

Yi-Huang Chang, R. W Hartel

380

Apparent

TABLE 3 Viscosity (mPa s) of High-Concentration Freeze-concentrated ferent Shear Rates (Rotational Viscometer)

Skim Milk at Dif-

TS (%)

25

30

35

Freezing point (“C)

-1.8

40

-2.3

-2.8

-3.4

-

-

Shear rate (se’) 0.132 0.33 0.66 1.32 2.64 6.36 I!!2 13:2

22.8 IfI0.5

53.7k2.1 52.5; 2.2 50.7f 1.7 50.4 f 2.4

22.9; 0.5

164+7 160f7 150+6 145_t6 1.37+5 -

936f71 885 f 66 822+61 742 A 56 -

skim milk was very sensitive to change of shear rate, whereas the viscosity of 25% freeze-concentrated milk was not. The variance of apparent viscosity of TS 40% freeze-concentrated skim milk was comparatively large. This might be due to crystallization of lactose from the liquid phase at such a high concentration. Further investigation is needed to explain this observation. Figure I shows a log-log plot of shear stress against shear rate for TS 40% freeze-concentrated skim milk. Rheograms for TS 2.5, 30 and 35% milks show similar behaviour over this temperature range. A non-linear regression analysis program (SAS Institute Inc., SASSTAT, Cary, NC) was used for determining yield freeze-concentrated

1

10 Shear

Rate (s-l)

Fig. 1. Shear stress versus shear rate for 40% TS freeze-concentrated temperatures (log scale).

skim milk at different

381

Flow properties of concentrated skim milk

stress. From this analysis, yield stress was equal to zero at all concentrations up to TS 40%. Table 4 shows the flow behaviour indices (n) and consistency indices (K) of high-concentration freeze-concentrated skim milk calculated from the power-law model and the Herschel-Bulkley model. Though the Herschel-Bulkley model provided high regression accuracy, the confidence intervals of the constants were wide and yield stresses were not significantly different from zero. The simple power-law model determined by linear regression rendered satisfactory results for the flow constants, assuming zero yield stress. From the results in Table 4, when the total solids content of freeze-concentrated skim milk was <25%, the milk could be regarded as a Newtonian fluid. When the total solids content was 30%, the milk exhibited non-Newtonian or pseudoplastic behaviour at the freezing point, though the deviation from Newtonian behaviour was quite small. The applicability of the Arrhenius model to the change in apparent viscosity with temperature over the range of concentrations is shown in Fig. 2. The Arrhenius equation fits well for both high- and low-concentration freeze-concentrated skim milk; the parameter fits (ye, and E,) are given in Table 5. Activation energy generally increased with concentration from 6.0 kcal/mol at 9.56% TS to 14.4 kcal/ mol at 40% TS.

TABLE4 Comparison TS

Temp. (“C)

(%I -

25

30

- 1.8 0 5 10 20 -2.3

40

5 10 20 ~ 2.8 0 5 10 20 -3.4 0 150 20

and Power-law Models concentrated Skim Milk

for High-concentration

Herschel-Bulkley model (non-linear regression) a<, (Pa)

0

35

of Herschel-Bulkley

K

0.00 If:0.00 0.23 + 0.01 0.00 * 0.00 0.21 f 0.01 O.OO+O.OO 0.16+0.01

I

-0.14kO.45 -0.08f0.30 -0.04kO.31 0.00 + 0.00 o.oo+o.oo -0.63AO.48 -0.56+0.53 -0.44fl.10 -0.15f0.76 0.00+0.07 -0.29+0.56 - 0.28 k 0.37 -0.24 + 0.42 -0.12kO.39 -0.13kO.52

-

0.64k0.23 0.51 kO.15 0.32kO.15 0.20 + 0.03 0.12+0.01 2.24kO.31 1.71 t-O.35 1.1340.62 0.57f0.44 0.24+0.03 8.0350.64 6.78 f 0.42 4.08 + 0.49 2.72kO.45 1.01 +O.ll

Freeze-

Power-law model (linear regression)

n

R2

K

n

R”

1.OOy- 0.02 1 .oo + 0.02 1.01 +0.02

0.999 0.999 0.999 -

0.23 A 0.01 0.21 k 0.01 0.16_tO.O1 -

1.OO* 0.02 1.OOk 0.01 1.01 kO.01 -

0.997 0.998 0.998

0.996 0.999 0.996 0.997 0.999 0.998 0.997 0.996 0.996 0.999 0.994 0.997 0.997 0.994 0.994

0.56 f 0.02 0.47 * 0.01 0.30 f 0.02 0.12+0.01 0.12+0.01 1.74 * 0.04 1.27kO.03 0.88 * 0.04 0.52 + 0.05 0.23 k 0.03 7.77 _t 0.18 6.5lkO.12 3.78 + 0.04 2.56 f 0.04 0.91 k 0.03

0.96 kO.01 0.97 * 0.01 0.99 f 0.03 1.01 kO.05 1.01 fO.O1 0.92 * 0.01 0.92_+0.01 0.92 5 0.02 0.94 + 0.05 1.05 + 0.07 0.90 * 0.02 0.90+0.02 0.90 If 0.02 0.94 & 0.02 0.93 + 0.02

0.996 0.997 0.991 0.975 0.998 0.995 0.994 0.988 0.955 0.937 0.991 0.994 0.995 0.990 0.992

0.92-tO.12 0.94_+0.10 0.9720.16 1.01 + 0.07 1.01+0.02 0.82-tO.05 0.81 kO.07 0.84kO.17 0.92kO.06 1.01 kO.04 0.82+0.10 0.82 + 0.08 0.84 + 0.08 0.891f:O.ll 0.89f0.11

‘Could not be determined with the instrument.

Yi-Huang Chang, R. W Hartel

382

--+

10%

-+---

15%

-+--

20%

-25% -30% -35% -40%

0.0034

0.0035

0.0036

0.0037

0.0038

l/T ("K -')

Fig. 2. Semi-log

plot of viscosity versus reciprocal temperature to show the temperature dependence of viscosity in terms of the Arrhenius equation.

Rheograms (0 vs. II) for freeze-concentrated and thermally evaporated skim milk at 35% TS are compared in Fig. 3. Results of a statistical analysis comparing freezeconcentrated and thermally evaporated skim milks are given in Table 6. The probability that the viscosity differed was not significant at the level of the 95% confidence interval over the temperature range in Table 6. Apparently, the viscosity of concentrated skim milk does not depend on the method by which it was manufactured, at least up to 35% TS. Comparison of the viscosities of TS 9.56% natural skim milk and TS 9.56% skim milks reconstituted from TS 1.5%, TS 35% freeze-concentrated and TS 35% therTABLE 5

Parameters

(E,

‘1I

(mPa s) 5.62 x 1.90 x 5.93 x 2.83 x 4.68 x 1.03 x 1.68 x

9.56 1.5 20 2s2 30’ 3s3 40” ’ rlu=

of the Arrhenius

rl ,

exp(E,,lRq.

“Shear rate 13.2 s-~ . “Shear rate 6.6 s- ‘. “Shear rate 1.32 s ‘.

IO-’ 1or5 10 (’ 10 ’ IO-’ IOPX 10P”

Model’ for Freeze-concentrated

Skim Milk

6,

R’

6.01 kO.17 6.90 f 0.84 7.85 f 0.52 7.33 f 0.46 10.0* 1.3 12.6k3.0 14.4 f 3.3

0.999 0.999 0.998 0.993 0.986 0.992 0.981

(kcallmol)

Flow properties of’concentruted

skim milk

383

F.C.(-2.8”C)

__-o

Evp.(-2.8”C)

1I n

F.C.(O”C)

l

Evp.(O”C)

-0-p

F.C.(S”C)

m

Evp.(S”C)

~~11

F.C.( 10°C)

~V~~~

Evp.( 10°C)

m--f

~ F.C.(ZO”C)

Kim-m Evp.(ZO”C)

5

10

15

20

Shear Rate (s‘ ‘) Fig. 3. Comparison of shear stresses for 35% TS thermally evaporated (Evp.) and freezcconcentrated (F.C.) skim milk at several shear rates and temperatures. evaporated skim milks is shown in Table 7. Because the viscosity at this concentration was quite low (in the range 1 to 4 mPa s), statistical analysis was done by comparing the efflux time through the glass capillary viscometer. Statistically, the viscosities of these four different skim milks were not identical even though they had the same percentage of total solids. However, the differences were actually quite small. The viscosities of skim milks reconstituted from TS 35% freeze-concentrated and TS 35% thermally evaporated products were slightly higher than the others at low temperature (-0.6, 5, 1O’C). The natural skim milk was the least viscous, although at higher temperature (20°C) differences in viscosity disappeared. At 20°C the viscosities of natural skim milk and reconstituted freeze-concentrated skim milks were statistically identical. Only the viscosity of mally

Comparison

of Apparent

TABLE 6 Viscosity of 35% TS Thermally Evaporated trated Skim Milk at Shear Rate 6.6 s ’

and Freeze-concen-

Evaporated skim milk (mPu s)

Freezeconcentrated skim milk (mPu s)

Prohuhiliy ’

(“C)

-2.8 0 5 10 20

152k3 111+2 72.9 f 0.7 47.2 * 1.3 23.9kO.4

14.5&6 112+2 73.4 f 2.1 46.7 f 0.8 24.8 _+0.2

0.067 0.852 0..352 0.174 0.053

Temp.

‘Probability

that the samples have the same viscosity.

Yi-Huang Chang, R. W Hartel

384

TABLE 7 Comparison of Apparent Viscosities (mPa s) of Natural Skim Milk (p,) and Reconstituted Skim Milks from 15% TS (p2) or 35% TS (Pi) Freeze-concentrated Skim Milks and from 35% TS Thermally Evaporated Skim Milk (Pi)’ Temp. (“C)

PI

P2

P.3

-0.6 5

3.7OkO.01” 2.97*0.01d 2.43 _t 0.01: 1.70 f 0.02’

3.73kO.01” 3.00_t0.01d 2.46 f 0.01” 1.71* 0.03’

3.81 kO.Olh 3.04*0.01’ 2.50k0.01h 1.78 f 0.03J

:II

P4

4.05 kO.01’ 3.29$0.01’ 2.75 + 0.0’ 1.94f0.11k

‘Note: values in any one row with different superscript letters differ significantly (at 95% confidence interval). reconstituted TS 35% thermally evaporated skim milk differed from that of the rest of the milks at this temperature. In all cases, viscosity increased slightly at lower temperatures. It is not clear that these small differences in viscosity would lead to noticeable differences in creaminess of reconstituted skim milks, based on sensory evaluation. Apparent viscosities of skim milks of different concentration at their freezing points and 20°C are plotted in Fig. 4. The apparent viscosity and variance for TS 40% milk at the freezing point were estimated by extrapolating the data for TS 40% milk to a shear rate of 6.6 SK’. The viscosity of milk at the freezing point increased rapidly from around 3 mPa s (at freezing point -0.6"C)for TS 9.56% to 600 mPa s (at freezing point -3.4”C) for TS 40% as a result of the freeze concentration process. Under this condition, heat and mass transfer at the ice-liquid milk interface is limited, with corresponding low ice crystal growth rates.

Fig. 4.

Apparent viscosity versus percentage

TS at freezing point and 20°C at shear rate

6.6 s-‘.

385

Flow properties of concentrated skim milk TABLE 8

Effect of Concentration Temp. (“C)

1; 20

Skim Milk Power model (eqn 7)

Exponential model (eqn 3) aI

R-

,I&? (Xl0 ‘)

a3

RJ

1.64+0.01

0.09 f 0.00

0.999

6.26 * 4.3 1

2.65 k 0.20

0.874

0.73 &0.74 f 0.09 0.07 0.44 + 0.06

0.13~0.01 0.12fO.01 0.12 f 0.0 1

0.966 0.978 0.96 I

7.29 l2.6k5.9 + 4.62 4.1372.60

2.50f0.19 2.22kO.15 2.47k0.19

0.877 0.907 0.861

‘II 0

on Viscosity of Freeze-concentrated

Both the exponential model [eqn (3)] and the power-law model [eqn (7)] could be used to explain the relation between apparent viscosity and concentration shown in Fig. 4. At 2o”C, the fit to the exponential relation is represented by the straight line in Fig. 4. The data at 0, 5, and 10°C also showed a similar relation. Table 8 lists the parameters for the exponential and power-law relations at 0, 5, 10 and 2O”C, as well as the value of R* in each case for the effect of concentration on viscosity. The exponential relation yielded a better fit than the power-law relation for the freeze-concentrated skim milk, with higher correlation coefficient R2. This result is consistent with the exponential model proposed by Langley & Temple (1985).

CONCLUSIONS During the freeze concentration process, the viscosity of skim milk increased about 200 times when TS increased from 9.56% to 40%. Maintaining rapid growth rate of ice crystals at such a high concentration might be difficult. The condensed skim milk also exhibited slight pseudoplastic flow behaviour. A higher agitation speed should be considered at high concentrations for the freeze concentration process, to reduce the viscosity of the fluid. Statistical analysis indicated that the viscosities of the TS 35% thermally evaporated skim milk and F.C. skim milk did not differ. On the other hand, the viscosity of TS 9.56% skim milk may depend on its history. Skim milk reconstituted from TS 35% freeze-concentrated skim milk and evaporated skim milk had a higher viscosity, though the actual differences were small. A subjective sensory analysis is needed to determine whether these viscosity differences would cause reconstituted milks to be considered creamier. REFERENCES Anon. (1991). Can skim milk taste like whole milk‘? New York Times, 14 Aug. AOAC

(1990). Solids (total)

in milk. In OfJicial Methods of Analysis: Food Composition; Vol.2. Association of Official Analytical Chemists, USA,

Additives; Natural Contaminants,

p. 807. Chowdhury, J. (1988). CPI warmup to freeze concentration. Chem. Eng., N.Y, 95(6), 24-31. Deshpande, S. S., Cheryan, M., Sathe, S. K. & Salunke, D. K. (1984). Freeze concentration of fruit juices. CRC Crit. Rev. Food Sci. Nuts:, 20(3), 173-247.

386

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Griffin, M. C. A., Price, concentrated, sterically milk. J. Colloid Intetface Hartel, R. W. & Espinel,

J. C. & Griffin, W. G. (1989). Variation of the viscosity of a stabilized, colloid: effect of ethanol on casein micelles of bovine Sci., 128, 223-229.

L. A. (1993). Freeze concentration

of skim milk. J. Food Eng., 20,

101-120.

Hayashi,

H. & Kudo N. (1989). Effect of viscosity on spray drying of milk [CD-ROM]. from: SilverPlatter File: FSTA: 90-06-P0129. Holdsworth, S. D. (1971). Applicability of rheological models to the interpretation of flow and processing behavior of fluid food products. J. Text. Stud., 2, 393-418. Jeurnink, T. J. M. & de Kruif, K. G. (1993). Changes in milk on heating: viscosity measurements. J. Dairy Res., 60, 139-1.50. Langley, K. R. & Temple, D. M. (1985). Viscosity of heated skim milk. J. Dairy Res., 52, Reports of Research Laboratory, Snow Brand Milk Products Co., 88, p. 53. Abstract

223-227.

Rao, M. A. (1977). Rheology of liquid foods-a review. J. Text. Stud., 8, 13.5- 168. Rao, M. A., Bourne, M. C. & Cooley, H. J. (1981). Flow properties of tomato concentrates. J. Text. Stud., 12, 521-538. Rao, M. A., Cooley, H. J. & Vitali, A. A. (1984). Flow properties of concentrated juices at low temperatures. Food Technol., 38(3), 113-l 19. Reddy, C. S. & Datta, A. K. (1994). Thermophysical properties of reconstituted milk during processing. J. Food Eng., 21(l), 31-40. Shi, Y., Liang, B. and Hartel, R. W. (1990). Crystallization of ice from aqueous solutions in suspension crystallizers. In Crystallization as a Separation Process, Symposium Series 438, eds A. S. Myerson and K. Toyokura. American Chemical Society, Washington, DC, pp. 316-28. Snoeren, T. H. M., Damman, A. J. & Klok, H. J. (1982). The viscosity of skim-milk concentrates. Netherlands Milk and Daity Journal, 36, 305-316. Van Mil, P. J. J. M. & Bouman, S. (1990). Freeze concentration of dairy products. Neth. Milk Dairy J., 44, 21-31.

Walstra, P. & Jenness,

R. (1984). Dairy Chemistry and Physics. Wiley, New York.